The present invention relates to a method and apparatus for dropout compensation of color television signals where both the luminance and chrominance components forming the dropout compensation signal are delayed during consecutive horizontal line intervals for a time differing from one horizontal line period by less than one cycle of the color subcarrier component which suitable for utilization in both analog and digital signal systems.
As it is well known in the art, television signal dropout compensators are utilized to replace a missing or degraded portion of the television signal which has "dropped out" due to an unpredictable instantaneous malfunction of the system. For example, when the television signal is recorded and subsequently played back from a recording medium, a dropout may occur due to diminutive defects of the recording medium. When such dropouts in the television signal occur, they introduce subjective distortion in the displayed television picture. Dropout compensators are utilized to eliminate the subjective distortion of dropouts from the television picture displayed to the viewer.
A comprehensive survey of prior art television dropout compensators is contained in a copending U.S. patent application Ser. No. 88,719 by B. Yeshwant Kamath, entitled "A Digital Filter and System for Processing Digital Composite Signals Incorporating the Filter", filed on Oct. 26, 1979, now U.S. Pat. No. 4,251,831, which is assigned to the assignee of ths application, which describes a digital color television signal dropout compensator. Generally, prior art dropout compensators utilize an RF envelope level detector that monitors the amplitude level of the modulated television signal carrier waveform to detect dropouts in the television signal. A compensatable dropout is manifested in a television signal as a momentary substantial reduction in the amplitude or loss of the television signal. A delay line is utilized to continuously delay the incoming original television signal. When a dropout in the original signal is detected, the delayed signal is applied as a dropout compensation signal to replace the dropout portion of the television signal information. More specifically a switch in the color television signal path is controlled to apply the incoming color television signal, or the delayed dropout compensation signal, respectively, in response to a control signal from the dropout detector. The color television signal is delayed by one horizontal line period and the chrominance component is processed to have its phase selectively altered on consecutive television lines. For example, in NTSC television signal systems, phase adjustment of the separated chrominance component of the dropout compensation signal is provided by delaying the chrominance component by two television line periods, or, alternatively, the phase of the separated chrominance component is reversed on consecutive television lines in various ways well known in the art.
However, there is a significant disadvantage characterizing the aforedescribed prior art techniques of dropout compensation. The original color television signal is separated into a luminance and chrominance component and each component is respectively delayed and processed in a separate signal path. Then, the separately processed signal components are recombined for use as a dropout compensation signal. When two or more consecutive horizontal lines contain a dropout, the previously processed and recombined composite signal is again separated and processed, as above described. This re-separation and re-processing is repeated for each one of the consecutive lines containing a dropout. The re-separation and re-processing causes line-to-line distortion and progressive degeneration of the dropout compensation signal results. Thus, when utilizing the above described prior art dropout compensation techniques, the obtained dropout compensation signal may become unacceptable whenever a few consecutive lines of the color television signal contain dropouts.